The present invention relates to a device that allows elements in sheet form to be printed using stamping.
The invention finds a particularly advantageous, although non-exclusive, application in the field of the manufacture of packaging intended for the luxury-goods industry.
It is known practice for texts and/or patterns to be printed by stamping, that is to say by using pressure to apply to a medium in sheet form, colored or metalized film taken from one or more stamping foils commonly known as metalized foils. In the industry, such a transfer operation is usually performed using a vertical platen press into which the press supports are introduced, sheet by sheet, while the stamping foils are fed continuously.
In a standard platen press, stamping is conventionally performed between a fixed platen extending horizontally, and a platen mounted so that it can move in a reciprocating vertical movement. Because this type of press is generally automated, conveyor means are provided to bring each sheet between the platens one by one. In practice, this means is usually a series of gripper bars, each of which in turn grasps a sheet at its frontal edge, before pulling it between the two platens when the latter have been parted sufficiently.
A stamping foil is itself schematically made up of a backing strip of polyester type, to which a pigmented layer is secured by a layer of wax. The external face of this pigmented layer is itself coated with a coat of hot-melt adhesive. As in the case of the sheets, the feed of stamping foils to the press is conventionally automated, by means of a drive system capable of unwinding each of said foils and feeding it in a clearly determined feed path which notably passes through the platen press. In general, such a foil feed system combines a series of diverting bars which are installed along the entire feed path to guide the progress of the foils, with a number of advance shafts which are positioned downstream of said path in order respectively to drive the movement of each of said foils.
In each machine cycle, a sheet is brought between the two platens, while the stamping foils are moved on and then likewise immobilized at the same point. The platen press is then closed. This closure presses the sheet and the foils between a plurality of forms and of matrices positioned facing one another on each of the platens respectively. Because the forms or the matrices are heated, the wax therefore melts and the hot-melt adhesive sets only at their regions of contact, thus transferring the pigments from the foils to the sheet in a given pattern.
In practice though, it is unfortunately found that, when the platen press is reopened after stamping, the various polyester backing strips have a natural tendency to remain stuck to the sheet.
In order to overcome this difficulty, it is known practice for separation between the stamping foils and the sheet to be forced by aiming a pressurized jet of air at where they are stuck. To do that, use is generally made of a blower which is fixed directly to the exit of the platen press, and which uses blowing nozzles positioned as close as possible to the plane in which the sheets move, that is to say directly in the gripper bar transfer region.
Such an arrangement does, however, have the disadvantage of entailing the use of gripper bars fitted with grippers at the top, that is to say with grippers which are configured to grasp each sheet substantially at the top face of the gripper bar. Now, it is known that this type of gripper is unable to guarantee optimal transport of the sheets through the platen press. The special form of the top grippers in fact causes each sheet to be moved some distance from the moving lower platen, that is to say without being supported so that it remains substantially horizontal. As it slows down, the sheet will therefore have a natural tendency to deform, thereby causing rumpling which is particularly detrimental to print quality.